Electric arcs

ABSTRACT

A source of electromagnetic radiation comprises two electrodes, means for connecting a voltage across the electrodes sufficient to maintain an arc discharge between them, and means responsive to a property of the discharge across the electrodes to adjust the separation of the electrodes to a value below that at which a mathematic minimum occurs in the voltage/separation curve.

United States Patent [1 1 Towlson 1 ELECTRIC ARCS [76] Inventor: Peter G. Towlson, 40 Old Gate Rd.,

Davington Ct., Favesham, England 22 Filed: Apr. 21, 1972 21 Appl. No.: 248,292

[30] Foreign Application Priority Data May 3, 1971 Great Britain 12,638/71 [51] Int. Cl. G2lh HOlj 61/96 [58] Field of Search [56] References Cited UNITED STATES PATENTS 2,724,790 11/1955 Embshoff et al 250/85 3,694,624 9/1972 Buchta 250/85 2,270.099 l/l942 Womer 250/85 2,952,762 9/1960 Williams et al 250/85 934,796 /l907 Davies 313/8 Primary Examiner-Harold A. Dixon Att0rneyStevens, Davis, Miller & Mosher [57] ABSTRACT A source of electromagnetic radiation comprises two electrodes, means for connecting a voltage across the electrodes sufficient to maintain an arc discharge between them, and means responsive to a property of the discharge across the electrodes to adjust the separation of the electrodes to a value below that at which a mathematic minimum occurs in the voltage/separation curve.

. 12 Claims, 5 Drawing Figures .v I i j i l, COMP/18A we PATENTEDUCI 23 I973 SHEET 2 [IF 3 a. ZEZESEw PATENTEU OCT 2 3 I973 SHEET 3 BF 3 ELECTRIC ARCS This invention relates to sources of electromagnetic radiation using an electrical discharge between two electrodes.

Earlier radiation sources of this type include the carbon-arc radiation source in which the carbon electrodes are initially brought together and then allowed to spring apart as the arc strikes between the electrodes. The arc is usually connected in series with a ballast resistance across a supply source, and the arc separation and the ballast resistance value arechosen to give arc currents and voltages which provide maximum light radiation. This conventional arc is known as the high intensity arc. The present invention is concerned with increasing the generation of heat.

In investigating how to modify the carbon arc discharge to provide large amounts of heat radiation, not necessarily accompanied by as large amounts of light radiation as formerly, l have provided control apparatus for the arc discharge which causes the electrode separation to be smaller than that previously used with similar conditions (such as electrode size) for the high intensity are. Higher efficiencies of heat generation are thereby obtained.

According to one aspect of this invention, there is provided a source of electromagnetic radiation comprising two electrodes, means for connecting a voltage across the electrodes sufficient to maintain an are discharge between them, and means responsive to a property of the discharge across the electrodes to adjust the separation of the electrodes to a value below that at which a mathematic minimum occurs in the voltage/- separation curve.

According to another aspect of the invention, there is provided a method of generating the electromagnetic radiation comprising connecting a voltage across two electrodes sufficient to maintain an arc discharge between them and adjustingthe separation of the electrodes in response to a property of the discharge to a value below that at which a mathematic minimum occurs in the voltage/separation curve.

An example of the invention will now be described with reference to the accompanying drawings in which:

FIG. 1A and 1B are a representation of a conventional are together with a representation of an arc produced according to the invention;

FIG. 2 shows a voltage/electrode separation characteristic of an arc;

FIG. 3 is a diagrammatic representation of an image furnace incorporating the invention; and 7 FIG. 4 is a circuit diagram of a component of the image furnace of FIG. 3.

The conventional D.C. high intensity arc has ashape as shown in FIG. 1 of the accompanying drawings with an elongated tail flame" 1 that extends over the anode 2 and points away from the cathode 3. Also within the tail flame" 1 are two bright areas or columns 4 and 5 that emanate from the electrodes 2 and 3 and are known as the anode and cathode columns respectively.

It has been now found that if the electrode separation is decreased, a point is reached at which the tail flame 1 together with the anode and cathode columns 4 and 5, respectively, disappears and is replaced by a bright conical flame or fireball 6 extending from the anode 2 towards the cathode 3 as also shown in FIG. 1.

FIG. 2 represents the relationship of the voltage across the arc to the separation between the electrodes. The high intensity are operates on the rising right hand part of the curve. As the separation increases in this region, the voltage increases, passing a knee and then rising more slowly until the arc is extinguished when the separation becomes too great. There is a mathematical minimum in the curve for the smallest separation in the high intensity region, and for separations below this point the voltage rises with decreasing separation to a maximum (which is below the maximum voltage in the high intensity region), after which the voltage again decreases with decreasing separation. In FIG. 2, the mathematical minimum refers to the point on the curve at the right-hand end of region I at which the voltage is less than the voltage at immediately adjacent points on the curve for greater and smaller separations. The arc is again extinguished when the separation between the electrodes becomes too small to support the are discharge. By means of highspeed photographic techniques it has been ascertained that the fireball 6 consists of particles of material that have been torn from the anode and are transferred to the cathode. Hence this form of arc is known as a transferred arc." The particles of matter torn from the anode are heated by the action of the arc and emit thermal radiation. 7

The heat radiation from the arc is measured by means of a black body radiation' colorim eter in which radiation is absorbed by the blackened interior of a water. cooled spherical cavity. Radiation fromthe arc is focussed on a small hole in the cavity housing. Thermo couples measure the temperature rise of the cooling water .passing through a tube soldered to the exterior of the cavity housing. Measurements taken from the same electrodes operated in the falling and rising portions of the. voltage/separation curve have shown that the'efficiency rises to a higher maximum whenoperating the arc inthefalling region of the curve. I

One application of arc radiation sources. of this type is in the arc image furnace, inwhich an image of the arc is focussed onto a sample to be heated. The discharge of FIG. 1 with a large tail flame and with anode and cathode columns which may all be unstable in position is not suitable for this purpose since the source of radiation is not concentrated, and is not fixed in position. The discharge according to the present invention shown in the lower part of FIG. 1 is more suitable for the arc image furnace. It has been proposed to concentrate the radiation source to the area between the electrodes by means of air jets discharged around the anode electrode towards the cathode, so that the radiation sources illustrated in the conventional high intensity are in FIG. 1 are prevented from rising above the region immediately between the electrodes. By operating the arc as shown in the lower part of FIG. 1, the source is containedwithout the need for air jets by operating in the falling voltage/separation region in the curve of FIG. 2.

Referring to FIG. 3, two carbon electrodes 31 and 32 are mounted coaxially in holders 33 and 34 so as to be separated by a distance s. Potentials are applied to the electrodes 31 and 32 so as to cause them to become the anode and cathode respectively of an electric arc struck between them. The operating potential'is applied to the electrodes 31 and 32 by means of a conventional DC. power supply which is not shown. The

anode 31 is positioned with one end 35 at one focus F of an ellipsoidal mirror 36. The cathode 32 projects through the pole of the mirror 36 to the vicinity of the end 35 of the anode 31. The anode 31 is maintained at its desired position by means of an infra-red sensing unit 37, to be described more fully later, that senses the position of the most intense part of the are between the electrodes 31 and 32, and operates to maintain this part of the are at the first focus of the mirror 36 by means of a servo motor 38 and a lead screw 39 upon which the holder 33 works.

The anode locating system 37 comprises a lens 46 that is positioned between two infra-red filters 47 and 48, and is so arranged as to form an image of the anode region of the are at a point between two infra-red sensitive photo-transistors 49 and 50. The photo-transistors 49 and 50 are connected in a dc bridge network 51, the circuit diagram of which is shown in FIG. 4. Any movement of the end 35 of the anode 31 from the first focal point F of the mirror 36 will cause a change in the position of the image formed by the lens 46 and result in an out-of-balance voltage being developed by the bridge 51 which is amplified by the dc. amplifier 52 and caused to operate the servo motor 38 and leadscrew 39 so as to hold the end 35 of the anode 31 in its proper position. The initial setting of the anode locating system 37 is by means of a hand wheel 53 and by a zero setting control (not shown) of the bridge 51, reference being made to a sighting system of cross-wires (also not shown) trained upon the knownposition of the focus F, of the mirror 36. A more sensitive control of the motors 38 and 40 can be achieved by frequency modulating the outputs of amplifiers 44 and 52, and applying the variable frequency signals to stopping motors. Such motors advance by a predetermined amount for each cycle of their variable frequency input.

The position of the cathode 32 is controlled to maintain the separation s of the anode 31 and cathode 32 at a desired value so that the arc will operate in a regime of its voltage/separation characteristic below that at which a mathematical minimum occurs, the preferred region being shown as I in FIG. 2, by a servo motor 40 and lead screw 41 similar to the servo motor 38 and lead screw 39. The cathode adjusting servo-mechanism is operated by means of an error voltage Ve that is developed by means of a comparator circuit 42 that compares the voltage Va across the arc with a reference potential Vref generated by a reference potential source 43, the error voltage Ve is amplified by means of a dc. amplifier 44. The arrangement is such that the direction and magnitude of the rotation of the servo motor 40 depends upon the slope of the voltage/separation characteristic and tends to maintain the arc in the region I of FIG. 2 having a negative slope. A reversing switch 45 is fitted to the input of the servo-motor 40 in case it should be desired to operate the are out of the negative slope region.

The cathode position, however, may be controlled in a similar manner to that used for the anode.

In use, the arc is first struck manually and the separation of the electrodes 31 and 32 is adjusted by hand until the desired operating conditions are achieved. If the desired operating conditions correspond to a point on the negative slope region of the curve in FIG. 2, the reversing switch is left unactuated, and the servomechanisms are brought into operation to maintain the desired conditions at that point on the curve. If the desired conditions relate to a point on the positive slope portion of the curve in FIG. 2 at separations below those of the negative slope region, the reversing switch 45 is actuated and the servo-mechanisms are brought into operation to maintain the operation of the arc in that positive slope region. It is also possible to operate the arc in the positive slope region of the curve in FIG. 2 at separations above the region I by adjusting the separation of the electrodes by hand to the appropriate value, actuating the reversing switch and bringing the servo-mechanism into operation; This region is however outside the scope of the present invention.

Care should be taken when operating the servomechanisms with a desired condition close to the maximum voltage point occuring at the lowest separation in the region I, as a transient may cause the conditions to move past the peak in the curve after which the control circuit will be unstable.

As an alternative to the manual arc formation method, an automatic mechanical device may be used in which the high current flowing between the electrodes in contact is used to operate a relay which operates a motor to separate the electrodes to the required distance. A further arc formation method is to apply a high frequency electric field to the region between separated electrodes. Another method of arc formation is to place a resistive body between the electrodes adjusted approximately to their working separation and to apply the current so that the current passing through the resistive body causes it to melt and evaporate, the vapour thereby assisting the striking of the arc. A ball of silver paper is a suitable resistive body.

Infra-red radiation proceeding from the mirror 36 passes through the second focus F of the mirror 36, which is arranged in conventional fashion to be the second focus F ofa second ellipsoidal mirror 36. A work piece 54 to be heated is arranged to be positioned at the first focus F, of the mirror 36'. The mirrors are pure ellipsoids as the source has its position accurately controlled by the circuits described above and there is no need to allow for variation of the arc position by designing the mirrors in a non-ellipsoid shape. The work piece 54 is surrounded by a hollow part-spherical radiation shield 55 made of a glass with a layer 56 of gold upon its outer surface. The work piece 54 is arranged at the geometric centre of the radiation shield 55, and the aperture 57 of the radiation shield 55 is such as to accept all the radiation reflected from the mirror 36 towards the workpiece 54. As the workpiece 54 is at the centre of the radiation shield 55, radiation from the workpiece 54 will be reflected back to the workpiece 54. The gold coating is itself provided with a protective coating, for example of silicon monoxide.

An alternative method of focussing the radiation from the arc includes the use of a lens system formed of material with very low absorption of infra-red radiation. Such a material is quartz which is substantially free ofwater and hydroxyl radicals. The lens system may be a single lens, or a number of thin lenses through which the radiation passes in succession. The lenses are suitably coated to prevent loss of efficiency by spurious reflections at their surfaces. Although the lens system absorbs some of the infra-red radiation, it has been found possible to use them continuously for up to twenty minutes with a beam of about 1 kilowatt without destruction of the lenses.

When it is required that the optical axis of the lens system should be parallel to the electrodes of the arc, the anode is arranged to be fed through a hole extending along the optical axis of the lens system in order to provide a compact optical layout.

The invention has application to conventional metallurgical processes such as welding, brazing, cutting, forging, freeforming and other purposes such as the deposition of metal coatings, crystal formation, and research into high temperature generally.

What I claim is:

l. A source of electromagnetic radiationcomprising two electrodes, means for connecting a voltage across the electrodes sufficient to maintain an arc discharge between them, and means responsive to a property of the discharge across the electrodes to adjust the separation of the electrodes to a value below that at which a mathematical voltage minimum occurs in the voltage/- separation curve.

2. A source as claimed in claim 1 wherein the responsive means is responsive to the infra-red radiation from the discharge.

3. A source as claimed in claim 1 wherein the responsive means includes means responsive to the voltage across the arc.

4. A source as claimed in claim 1 comprising means to focus an image of the arc discharge at a remote point, said means comprising a reflector of gold coated upon a supporting substrate.

5. A source as claimed in claim 1 comprising means to focus the arc discharge at a remote point, said means comprising a lens of water-free and hydroxyl-free material.

6. A source as claimed in claim 5 wherein the means to form an image of the arc discharge is formed with a hole through which one of said electrodes extends.

7. A method of generating electromagnetic radiation comprising connecting a voltage across two electrodes sufficient to maintain an arc discharge between them and adjusting the separation of the electrodes in response to a property of the discharge to a value below that at which a mathematical voltage minimum occurs in a voltage/separation curve.

8. A method as claimed in claim 7, wherein the property of the discharge is infra-red radiation from the discharge.

9. A method as claimed in claim 7 wherein the property of the discharge is the voltage between the electrodes.

10. A method as claimed in claim 7 comprising focusing an image of the arc discharge at a remote point by refracting the radiation from the arc discharge through a lens of water-free and hydroxyl-free material.

11. A method as claimed in claim 7 comprising focusing an image of the arc discharge onto a remote point by reflecting radiation from the arc discharge onto said point by means of a reflector of gold coated upon a substrate.

12. A method as claimed in claim 11 comprising feeding one of the electrodes through an aperture formed in the device for focusing the image of the arc discharge at a remote point. 

1. A source of electromagnetic radiation comprising two electrodes, means for connecting a voltage across the electrodes sufficient to maintain an arc discharge between them, and means responsive to a property of the discharge across the electrodes to adjust the separation of the electrodes to a value below that at which a mathematical voltage minimum occurs in the voltage/separation curve.
 2. A source as claimed in claim 1 wherein the responsive means is responsive to the infra-red radiation from the discharge.
 3. A source as claimed in claim 1 wherein the responsive means includes means responsive to the voltage across the arc.
 4. A source as claimed in claim 1 comprising means to focus an image of the arc discharge at a remote point, said means comprising a reflector of gold coated upon a supporting substrate.
 5. A source as claimed in claim 1 comprising means to focus the arc discharge at a remote point, said means comprising a lens of water-free and hydroxyl-free material.
 6. A source as claimed in claim 5 wherein the means to form an image of the arc discharge is formed with a hole through which one of said electrodes extends.
 7. A method of generating electromagnetic radiation comprising connecting a voltage across two electrodes sufficient to maintain an arc discharge between them and adjusting the separation of the electrodes in response to a property of the discharge to a value below that at which a mathematical voltage minimum occurs in a voltage/separation curve.
 8. A method as claimed in claim 7, wherein the property of the discharge is infra-red radiation from the discharge.
 9. A method as claimed in claim 7 wherein the property of the discharge is the voltage between the electrodes.
 10. A method as claimed in claim 7 comprising focusing an image of the arc discharge at a remote point by refracting the radiation from the arc discharge through a lens of water-free and hydroxyl-free material.
 11. A method as claimed in claim 7 comprising focusing an image of the arc discharge onto a remote point by reflecting radiation from the arc discharge onto said point by means of a reflector of gold coated upon a substrate.
 12. A method as claimed in claim 11 comprising feeding one of the electrodes through an aperture formed in the device for focusing the image of the arc discharge at a remote point. 